1. Field of the Invention
This invention relates generally to absorption spectroscopy; and more particularly to a waveguide and apparatus for the detection and measurement of trace gases, preferably absorption spectroscopyxe2x80x94and most preferably by wavelength-modulation absorption spectroscopy.
2. Related Art
Much of the development of today""s pollution monitoring equipment was carried out after passage of the Clean Air Act. At that time electrooptical technology was in its infancy in comparison with current capabilities. One of the primary disadvantages of optical techniques for gas detection and measurement is that instruments tend to be large and complex.
Analyzers that meet the EPA designated reference method require specialized techniques for specific gases. For example, N02 is detected by chemiluminescence; CO by gas correlation; O3 by absorption; and SO2 by fluorescence. There has been no single effective methodology that could be used to monitor key pollutants of interest.
Current analyzers are typically packaged one to a 48 cm (19 inch) rack-mounted chassis. These analyzers typically weigh nearly 25 kg, including the required sample handling components, power supplies and electronics. Ambient air monitoring stations tend to involve buildings to enclose the analyzers and other sophisticated support equipment, and dramatically increase the cost associated with pollution assessment.
Similar problems exist for continuous emission monitoring systems used to evaluate flue gas from coal- and gas-fired electrical generation facilities. In many cases analyzers are mounted inside large dedicated shelters. Samples of the flue gas are transported over long distances in heated sample lines from the stack to the shelters.
Ultraviolet absorption bands of gases tend to be relatively broad due to the nature of the changes experienced by the molecules. There are significant overlaps in spectral features of different molecules.
Spectral selectivity is required to overcome the inherent problem of interference. Lamp inefficiencies also require high heat dissipation. Several UV instruments have been used for pollution monitoring; however, these systems are relatively large and require complicated spectral deconvolution algorithms to eliminate interference.
Both infrared and ultraviolet techniques require relatively long light/sample interaction paths to achieve sensitive detection capabilities. This has been achieved using multipass cells that reflect the light repeatedly over a folded path inside a container that holds the sample.
To achieve adequate pathlengths these cells are often large ( greater than 1000 cc) in volume. Noise and drift associated with the measurements impose strict design criteria to maintain alignment of the cells. Cell temperatures are usually controlled to maintain stability, and large pumps are required to achieve rapid pneumatic response.
A prior-art device that potentially provides a more compact gas analyzer is shown in U.S. Pat. No. 5,341,214 to Wong. This patent discloses an inherently rigid cylindrical waveguide/chamber having an internal blackbody radiation source mounted within one end, and two infrared detectors mounted within the opposite end of the chamber.
The radiation is reflected at the walls of the chamber to increase the light path. Each detector includes an optical filter; one filter defines a spectral band that coincides with the infrared absorption of the gas to be measured, and the other defines a nonabsorption bandpass.
The signals from Wong""s detectors are processed to provide a ratio related to the concentration of the gas in the sample. The chamber includes a membrane-covered inlet aperture and an outlet aperture to permit flow through the chamber.
A device utilizing this technology is likely suited to detect only a high concentration of a gas. Short optical pathlength limits sensitivity for low concentrations, and the low gas flow limits the response time.
Another prior-art device is shown in U.S. Pat. No. 5,384,640, also to Wong. This patent discloses an evidently rigid cylindrical waveguide/chamber having an integral laser positioned within one end of the chamber, and an integral detector positioned within the opposite end.
Apertures in the wall of the chamber enable flow of gas through the chamber. The cylinder can be partitioned into successive chambers for the detection of different gases. The chamber is relatively large, having a diameter greater than the diameters of the laser and the detector, and therefore as a practical matter is quite limited in optical length and also optical efficiency. From the smallest commercially available laser and detector packaging, it can be seen that the inside diameter of Wong""s chamber is at least 5 mm.
Consequently a device based on this patent too appears limited to measuring only high concentrations of a gas, and only by direct absorption (not by wavelength-modulation absorption). The optical length and resulting sensitivity are further limited by partitioning the chamber for the detection of multiple gases.
U.S. Pat. No. 5,696,379 to Stock discloses a prior-art infrared absorption device incorporating a curved tubular waveguide having a plurality of random gas inlets along the length of the waveguide and having an internal radiation source within the waveguide. A second radiation source is positioned within the waveguide near detectors and utilized to compensate for deterioration drift and temperature drift of the detectors. A device based upon this patent is believed to have a relatively large-diameter, low-efficiency waveguide with a relatively short optical path, and consequently to have limited sensitivity and slow response times between measurements.
There have been significant recent developments in hollow optical waveguides for energy delivery in the infrared regions (0.8 to 12 p) of the spectrum. Such improvements are disclosed in U. S. Pat. No. 5,440,664; 5,567,471; and 5,815,627 to Harrington et al., and are assigned to Rutgers, The State University of New Jersey. This technology provides waveguides for preservation of good transverse coherence of input infrared laser radiation, and that transmit substantial power of such radiation with low attenuation. The waveguide typically comprises a small-diameter, thin-wall silica glass tube, a protective outer coating, a reflective layer on the inner surface of the tube, and a dielectric coating on the exposed surface of the reflective layer.
Heretofore, this new waveguide technology has not been effectively applied and packaged for compact absorption spectroscopy.
There have also been significant recent developments of semiconductor diode lasers and the quantum cascade laser. Such improvements represent new compact tunable near- and midinfrared coherent light sources that can operate at room or thermoelectric control temperatures, and can be customized for specific wavelengths.
The explosive growth and miniaturization of electronics provides high-speed analog-to-digital converters and high-throughput digital signal processing capabilities, that have heretofore not been applied and incorporated into systems for compact, accurate and reliable absorption spectroscopy.
As shown above, the related art remains subject to significant problems and has left room for considerable refinement.
The present invention introduces such refinement, and provides a novel waveguide and optical gas-sensor apparatus that is compact, lightweight, durable, extremely sensitive, accurate and reliable, and has a rapid response time.
The invention has several independently usable facets or aspects, which will now be introduced. In preferred embodiments of a first of its main aspects or facets, the present invention is a waveguide for holding a gaseous specimen for spectral analysis.
The waveguide includes a hollow elongated tube having an interior with inside diameter not exceeding two millimeters (2 mm), a radiation inlet and a radiation outletxe2x80x94and also having a wall with a substantially smooth reflective inner surface, adapted for guiding radiation along the interior. In addition the waveguide includes some means for passage of the gaseous specimen through the wall into the interior of the tube. For purposes of breadth and generality in discussing the invention, these means may be called simply the xe2x80x9cpassage meansxe2x80x9d.
The foregoing may be a description or definition of the first facet or aspect of the present invention in its broadest or most general form. Even as couched in these broad terms, however, it can be seen that this facet of the invention importantly advances the art and resolves certain of the previously outlined problems.
In particular, because of its small diameter the waveguide can be curved or bent and thereby made extremely compact in any number of very favorable configurationsxe2x80x94a very long tube being configurable, in particular, into a very small volume. Useful arrangements include disorderly or chaotic stacks, as well as geometrical forms that are regular.
In contrast with the previously discussed larger-diameter, inherently rigid Wong devices, for instance, this first aspect of the invention can provide an extremely long optical path for very high sensitivity. No correspondingly bulky or heavy enclosure, frame, etc. is required.
Furthermore the small diameter imparts to the waveguide a degree of resilience, even after its fabrication into a module of an apparatus, and thereby an immunity to common kinds of damage that derive inherently from rigid and accordingly brittle constructions. Hence the waveguide of the first aspect of the invention is immediately conducive to important advances in provision of a fully portable, small, lightweight and also very robust gas spectrometer.
Although the first major aspect of the invention thus significantly advances the art, nevertheless to optimize enjoyment of its benefits preferably the invention is practiced in conjunction with certain additional features or characteristics. In particular, preferably the passage means include a first portion of the tube, having at least one perforated opening through its wall, adapted for receiving the gaseous specimen. The passage means preferably also include a second portion of the tube, also having at least one perforated opening through its wall, adapted for exhausting the gaseous specimen.
In this case it is furthermore preferable that the tube be shaped into a coil having at least one loop, and that the first portion and the second portion be on opposed sides of the coil. In this way the tube is adapted to provide two or more paths by which the gaseous specimen can flow, in the tube. As will be made more clear shortly, the tube thus provides two or more gas-flow paths even if the coil has only one loop.
Also in this case preferably the waveguide includes a housing supporting the tube, and having a gas entry chamber and a gas outlet chamber. The inlet chamber encloses the first portion of the tube and is adapted for receiving the gaseous specimen, and the exhaust chamber encloses the second portion of the tube and is adapted for exhausting the gaseous specimen.
Additional preferences related to the preferred coiled configuration of the waveguide include these:
the tube has an inner diameter in arrange of about 250 to 2000 xcexcm and a length in a range of about three to twenty meters;
the coil has a radius of curvature in the range of about five to twenty centimeters and a number of loops in a range of about eight to sixty-five;
in the first portion, each of the loops has at least one perforated opening therein, of diameter in a range of about ten to one hundred micrometers; and
in the second portion, each of the loops has at least one perforated opening therein, of diameter in the range of about ten to one hundred micrometers.
Still further preferences related to the coiled form of the invention include, for the tube, inner diameter of about one-half millimeter and length of about tan meters; and for the coil, a radius of about five centimeters and loop count of about thirty-one and a half. In both the first and second portions, preferably each loop has plural perforated openings of diameter about fifty micrometers.
In preferred embodiments of a second major independent facet or aspect, the invention is apparatus for detecting and determining a concentration of at least one gas within a gaseous specimen by radiation-absorption spectroscopy. The apparatus includes a coiled hollow waveguide having a radiation inlet and a radiation outlet, and the waveguide is adapted for guiding radiation along its interior.
The apparatus also includes some means for providing a plurality of paths for flow of the gaseous specimen within the waveguide. Again for purposes of breadth and generality these means will be called the xe2x80x9cproviding meansxe2x80x9d.
Also included are some means for projecting radiation along the interior of the waveguide to irradiate the gaseous specimen. These means, again for generality and breadth, will be called the xe2x80x9cprojecting meansxe2x80x9d.
The apparatus still further includes some means for analyzing the radiation emerging from the gaseous specimen to detect and determine the concentration of at least one gas in the specimen. These means, as before, will be called the xe2x80x9canalyzing meansxe2x80x9d.
The foregoing may represent a description or definition of the second aspect or facet of the invention in its broadest or most general form. Even as couched in these broad terms, however, it can be seen that this facet of the invention importantly advances the art.
In particular, while this aspect of the invention is capable of providing plural gas-flow paths across the coiled waveguide, the optical-absorption path can most naturally extend along the entire waveguide for a single continuous distance. Therefore the invention enjoys an extremely favorable relationship between rapidity of gas changeover (for very quick response to changes in gas constituents) and a long optical path for extremely high optical-absorption sensitivity.
Although the second major aspect of the invention thus significantly advances the art, nevertheless to optimize enjoyment of its benefits preferably the invention is practiced in conjunction with certain additional features or characteristics. In particular, preferably the means for providing a plurality of paths include the coiled waveguide having at least one complete loop.
In this preferred configuration, a first portion of each loop and a second portion of each loop are at opposed sides of the coiled waveguide. The first portion has at least one perforated opening for receiving the specimen, and the second portion has at least one perforated opening for exhausting the specimen.
Accordingly the plurality of paths includes two half-loop paths across each loop, respectively. The second portion, in this preferred form of the invention, has an exhaust pump for exhausting the specimen, which impels flow of the specimen through the waveguide.
Another preference is that the projecting means include a laser disposed to project radiation into the inlet of the waveguide. Here the laser preferably includes a semiconductor laserxe2x80x94and most preferably a semiconductor quantum cascade laser.
Yet another preference is that the analyzing means include a photodiode disposed to receive the radiation emitted from the outlet of the waveguide, and signal processing means for analyzing the output of the photodiode. Still another basic preference is that the apparatus include some means for displaying the output of the analyzing means, and also some means for control of operation of the apparatus by an operatorxe2x80x94here called the xe2x80x9cdisplaying meansxe2x80x9d and xe2x80x9ccontrol meansxe2x80x9d respectively. In this latter case the displaying means and the control means are adapted for operation at a location remote from the source of the gaseous specimen.
In preferred embodiments of a third major independent facet or aspect, the invention is apparatus for detecting and determining a concentration of two or more respective specific gases within a gaseous mixture specimen. The apparatus includes a coiled hollow waveguide having a radiation inlet and a radiation outlet, and is adapted for guiding radiation along the interior.
Also included are some means for providing a plurality of paths for flow of the gaseous specimen within the waveguide. For reasons outlined earlier these means will be called the xe2x80x9cproviding meansxe2x80x9d.
The apparatus additionally includes some means for sensing and recording spectra for the plural specific gases of the gaseous specimen. These will be called the xe2x80x9csensing and recording meansxe2x80x9d.
Yet further included are some means for projecting radiation along the interior of the waveguide, to be absorbed by each of the plural specific gases therein; and also some means for analyzing the radiation emerging from the gaseous specimen. These means will be called the xe2x80x9cprojecting meansxe2x80x9d and the xe2x80x9canalyzing meansxe2x80x9d, respectively.
The foregoing may represent a description or definition of the third aspect or facet of the invention in its broadest or most general form. Even as couched in these broad terms, however, it can be seen that this facet of the invention importantly advances the art.
In particular, this aspect of the invention provides the capability, in a very economical instrument, for determining the presence and quantities of two or more gasesxe2x80x94and also, perhaps more interestingly, for characterizing relatively complicated conditions. That is, the invention is not limited to quantifying levels of two or more gases as such.
It also can be configured to gauge favorable or unfavorable conditions for a mechanism (e. g. emissions control in a motor vehicle), or for a factory (e. g. quality control in a petroleum-cracking process stream). It can serve in any one of a very great variety of confined workspaces ranging from sewer lines to mines to submarines (e. g. monitoring for explosive or other hazardous atmospheres).
Although the third major aspect of the invention thus significantly advances the art, nevertheless to optimize enjoyment of its benefits preferably the invention is practiced in conjunction with certain additional features or characteristics. In particular, preferably the projecting means include a tunable laser for projecting radiation into the waveguide inletxe2x80x94more preferably a semiconductor laser, and ideally a semiconductor quantum cascade laser.
A variety of selectable preferences for the plural-gas analyzing forms of the invention will now be mentioned. It will become clear that these preferences respectively correspond to different industrial or other applications, such as suggested above.
In one such preference the plural respective gases include at least one of NH3, CO2 and H2O; and here the laser is tunable within the near-infrared spectrum at about 1.54 xcexcm. In other preferences the plural gases within the gaseous mixture specimen include at least one of these gases:
in one case, NH3, CO2, CH4, H2O, and NO;
in another case, NO, NO2, SO2, NH3, CO, C02, and H2O;
in yet another case, HF, O2, H2O, and O3; and
in still another case, NO, NO2, CH4, BTX and VOCs.
In preferred embodiments of a fourth major independent facet or aspect, the invention is a handheld self-contained portable apparatus for detecting and measuring at least one gas within a gaseous specimen by radiation-absorption spectroscopy. The apparatus is for use with a portable source of energy for powering the apparatus.
It includes a coiled waveguide having a radiation inlet and a radiation outletxe2x80x94and adapted for guiding radiation along its interior. It also includes some means for providing flow of the specimen within the interior of the waveguidexe2x80x94the xe2x80x9cflow providing meansxe2x80x9dxe2x80x94and some means for projecting radiation along the interior of the waveguide to irradiate the gaseous specimen, the xe2x80x9cprojecting meansxe2x80x9d.
Also included are some means for analyzing the radiation emerging from the gaseous specimenxe2x80x94the xe2x80x9canalyzing meansxe2x80x9dxe2x80x94to detect and determine the concentration of at least one gas in the specimen. Also included in the apparatus are some meansxe2x80x94the xe2x80x9chousing meansxe2x80x9dxe2x80x94for enclosing the waveguide, the projecting means, the analyzing means, the path-providing means and the flow-providing means.
The foregoing may represent a description or definition of the fourth aspect or facet of the invention in its broadest or most general form. Even as couched in these broad terms, however, it can be seen that this facet of the invention importantly advances the art.
In particular, this form of the invention provides portability and compactness in a full-capability gas analyzer. These features promise to be extremely valuable for sensitive measurements under difficult field conditions xe2x80x94whether in remote, rugged areas or for mobile operation in large cities, or in other kinds of challenging environments such as outlined earlier.
Although the fourth major aspect of the invention thus significantly advances the art, nevertheless to optimize enjoyment of its benefits preferably the invention is practiced in conjunction with certain additional features or characteristics. In particular, preferably the apparatus has overall dimensions not exceeding about twenty-five centimeters by thirty centimeters by eight centimeters (25 by 30 by 8 cm).
Preferably its weight is about three kilograms (3 kg) at most. Preferably the projecting means include a laser adapted to project radiation into the inlet of the waveguide; and the analyzing means include a photodiode adapted to receive the radiation emitted from the outlet of the waveguide, as well as signal processing means adapted for analyzing the output of the photodiode.
Another preference is that the housing means include a housing supporting the waveguide in coiled loops. Here the housing has an inlet chamber enclosing a first portion of the loops and receiving the gaseous specimen therein, and has an exhaust chamber enclosing a second portion of the loops and exhausting the gaseous specimen therefrom. In this case the invention also includes some means for controlling the apparatus, and some means for displaying its resulting analysis.
In preferred embodiments of a fifth major independent facet or aspect, the invention is a method for adapting an elongated hollow waveguide for flow of a gaseous specimen through the wall of the waveguide. The method includes the step of providing an elongated hollow waveguide that is flexible.
It also includes the step of forming at least one perforated opening through a first portion of the wall of the flexible waveguide for entry of the gaseous specimen. Another included step is forming at least one perforated opening through a second portion of the wall of the flexible waveguide for exhaust of the gaseous specimen.
The foregoing may represent a description or definition of the fifth aspect or facet of the invention in its broadest or most general form. Even as couched in these broad terms, however, it can be seen that this facet of the invention importantly advances the art.
In particular, this novel method is key to very efficiently preparing a module that can serve as both a high-quality waveguide and a flow cell. Although the fifth major aspect of the invention thus significantly advances the art, nevertheless to optimize enjoyment of its benefits preferably the invention is practiced in conjunction with certain additional features or characteristics. In particular, the several apparatus aspects and preferences introduced above are applicable in regard to this method form of the invention as well.
In preferred embodiments of a sixth major independent facet or aspect, the invention is a method for adapting an elongated hollow waveguide, for a plurality of paths of flow of a gaseous specimen through the wall of the waveguide. The method includes the step of securing the waveguide into a coil having at least one loop.
A gas entry portion of each loop and a gas exhaust portion of each loop are at opposed sides of the coil. The method also includes the step of forming at least one perforated opening through the wall of substantially each loop in the entry portion, for entry of the gaseous specimen.
Another step is forming at least one perforated opening through the wall of substantially each loop in the exhaust portion, for exhaust of the gaseous specimen. In this way, substantially each of the perforated loops provides twoxe2x80x94or roughly twoxe2x80x94half-loop paths for flow of the gaseous mixture from the entry portion to the exhaust portion.
The foregoing may represent a description or definition of the sixth aspect or facet of the invention in its broadest or most general form. Even as couched in these broad terms, however, it can be seen that this facet of the invention importantly advances the art.
In particular, this aspect of the invention serves very well for efficiently preparing a module that can serve as both a high-quality waveguide and high-throughput flow cell. Although the sixth major aspect of the invention thus significantly advances the art, nevertheless to optimize enjoyment of its benefits preferably the invention is practiced in conjunction with certain additional features or characteristics.
In particular, there are several selectable preferences as to the two forming steps. These preferences include employing the step of laser drilling, or chemical etching, or photolithography, or electrostatic discharge.
Another preference is that the forming steps include forming the perforated openings having a diameter about one-tenth the inner diameter of the waveguide. A preferred result of the method is a waveguide produced by the specified method. That waveguide preferably has an inside diameter of about one-half millimeter and perforated openings of diameter about fifty micrometers.
In preferred embodiments of a seventh major independent facet or aspect, the invention is apparatus for detecting and measuring at least one gas within a gaseous specimen by radiation-absorption spectroscopy. The apparatus includes a housing.
It also includes an elongated coiled hollow flexible waveguide arranged within the housing. The waveguide has a radiation inlet adapted for receiving radiation, and in addition a radiation outlet adapted for emitting the received radiation.
The housing has a gas entry chamber enclosing a first portion of the waveguide and adapted for receiving the gaseous specimen. It also has a gas exhaust chamber enclosing a second portion of the waveguide and adapted for exhausting the gaseous specimen. The first portion and the second portion are on opposed sides of the coiled waveguide.
The first portion includes part of at least one loop of the waveguide. At least one perforated opening is defined in that part of the waveguide, so that the interior of the waveguide is in communication with the gas entry chamber.
The second portion includes part of at least one loop of the waveguide. At least one perforated opening is defined in that part of the waveguide too, so that the interior of the waveguide is in communication with the exhaust chamber. The gaseous specimen flows from the inlet chamber through the waveguide into the exhaust chamber.
An exhaust pump is connected to the exhaust chamber for providing flow of the specimen through the waveguide. A source is adapted for projection of electromagnetic radiation into the inlet of the waveguide, and a detector is adapted for receiving the radiation emitted from the outlet of the waveguide. The apparatus also includes a signal processor for analyzing the output of the detector to identify and determine the concentration of at least one gas within the gas specimen.
The foregoing may represent a description or definition of the seventh aspect or facet of the invention in its broadest or most general form. Even as couched in these broad terms, however, it can be seen that this facet of the invention importantly advances the art.
In particular, this apparatus combines most of the beneficial features of the previously introduced aspects of the invention. It accordingly represents a very powerful and sophisticated tool for gas analysis with a compact, lightweight, robust, and high-gas-throughput sensor.
Although the seventh major aspect of the invention thus significantly advances the art, nevertheless to optimize enjoyment of its benefits preferably the invention is practiced in conjunction with certain additional features or characteristics. In particular, the several preferences described in earlier passages of this section are applicable here as well.
All of the foregoing operational principles and advantages of the present invention will be more fully appreciated upon consideration of the following detailed description, with reference to the appended drawings, of which: